gabble_muc_channel_init: initialize self->priv correctly
gabble_muc_channel_init: initialize self->priv correctly

It was uninitialized, which had never been significant so far because
all of muc-channel.c redundantly calls GABBLE_MUC_CHANNEL_GET_PRIVATE
again.

community-ssu/telepathy-gabble,community-ssu/telepathy-gabble,community-ssu/telepathy-gabble,community-ssu/telepathy-gabble